Processing black-surround screens

ABSTRACT

The faceplate of a color cathode-ray tube is covered with a sensitized aqueous solution of polyvinyl alcohol (PVA) and exposed through the shadow mask to establish dot images in the selected elemental areas of the screen that are to receive phosphor materials. These images are developed by washing the screen with water to remove the unexposed portions of PVA and then the screen is baked to render the dot images opaque to actinic radiation. Thereafter, the screen is coated with a similar photosensitive material including a light-absorbing pigment and is exposed from the side opposite that which has been coated. Washing with water removes all portions of the second layer which overlie the opaque PVA dot images, confining the pigment to the portions of the screen which surround the dot images. Thereafter, a thin film of a sensitized aqueous solution of PVA is applied to the faceplate and subjected to a flooding beam of ultraviolet light to render the film insoluble in water and form a surface hardened coating over the pigment to lock it securely against erosion. Following this, three sets of deposits of different color phosphors are applied over the developed dot images after which the faceplate is ultimately baked out.

United States Patent [72] Inventors Constantin S. Szegho;

Sam H. Kaplan, both of Chicago, Ill. [21] AppLNo. 773,832 [22] Filed Nov. 6, 1968 [4S] Patented Oct. 26, 1971 [73] Assignee Zenith Radio Corporation [54] PROCESSING BLACK-SURROUND SCREENS 3 Claims, 7 Drawing Figs.

[52] U.S.Cl 96/36.], 96/29, 96/36, 96/451, 117/335, 313/109 [51] 1nt.C| H0lj l/54, HOlj 9/22 [50] FieldofSearch ..96/36,36.1, 29,351,451; 117/335; 313/109 [56] References Cited UNITED STATES PATENTS 2,840,470 6/1958 Levine 96/361 2,992,919 7/1961 Beeleretal.... 96/361 3,146,368 8/1964 Fiore et a1 96/36.l 3,317,319 5/1967 Mayaud.. 96/36.] 3,330,682 7/1967 Tamura 1l7/33.5 3,365,292 1/1968 Fiore etal 96/36 llb' Primary Ei'amineF-William D. Martin Assistant Examiner-William R. Trenor Arlorney- Francis W. Crotty ABSTRACT: The faceplate of a color cathode-ray tube is covered with a sensitized aqueous solution of polyvinyl alcohol (PVA) and exposed through the shadow mask to establish dot images in the selected elemental areas of the screen that are to receive phosphor materials. These images are developed by washing the screen with water to remove the unexposed portions of PVA and then the screen is baked to render the dot images opaque to actinic radiation. Thereafter, the screen is coated with a similar photosensitive material including a light-absorbing pigment and is exposed from the side opposite that which has been coated. Washing with water removes all portions of the second layer which overlie the opaque PVA dot images, confining the pigment to the portions of the screen which surround the dot images. Thereafter, a thin film of a sensitized aqueous solution of PVA is applied to the faceplate and subjected to a flooding beam of ultraviolet light to render the film insoluble in water and form a surface hardened coating over the pigment to lock it securely against erosion. Following this, three sets of deposits of different color phosphors are applied over the developed dot images after which the faceplate is ultimately baked out.

IlllliillllllWW/IY/l/A PROCESSING BLACK-SURROUND SCREENS CROSS-REFERENCE TO RELATED APPLICATION BACKGROUND OF THE INVENTION The subject invention is addressed to the processing of a black-surround screen for a color picture cathode-ray tube.

Such a screen has interleaved sets of deposits of different phosphor materials separated from one another by a light-absorbing pigment. Structures of this type and processes for preparing them are described and claimed in US. Pat. No. 3,146,368 issued Aug. 25, i964 and US. Pat. No. 3,365,292 issued Jan. 23, 1968, both in the name of Joseph P. Fiore et al. and assigned to the assignee of the present invention.

A screen having the black-surround feature may be formed of phosphor deposits that are either stripes or dots but, since the latter is more frequently used, the description will continue on the assumption that the screen under process has dot triads each of which has a dot of green, a dot of blue, and a dot of red phosphor.

In preparing such a screen the phosphor dots may be applied first and the spaces between them subsequently covered with a light-absorbing pigment or the pigment may be applied first, followed by the formation of the dot triads. The latter has attractive features pointed out in the concurrently filed Kaplan application. The invention will be described initially in that environment, being employed to prevent erosion of the black-surround material in the subsequent screening steps necessary to the forming of dot triads. The objective, of course, is to confine the pigment strictly to the screen positions that surround the phosphor dots and thus avoid cross contamination. Accordingly, it is an object of the invention to provide a novel method of forming a black-surround screen for a color cathode-ray tube.

It is a particular object of the invention to provide a process for fonning such a screen which minimizes the possibility of cross contamination of the phosphor deposits with the blacksurround material.

SUMMARY OF THE INVENTION =fl'he method of the invention facilitates forming on the sets of phosphor material. The method contemplates applying one of the patterns of materials to assigned and spaced elemental areas of the screen. For example, a material that has light-absorbing capabilities may be applied to portions of the screen except for the elemental areas thereof that are to receive deposits of phosphor materials. Next, there is formed over the pattern of light-absorbing material an overcoat of a nontacky film of volatilizable material that is insoluble in a particular solvent, such as water. Thereafter, the entire screen is coated with a composition including material for the other pattern and the aforesaid solvent. Then, the coating is removed from those portions of the screen that are not to retain this coating material such as those elemental areas that have been covered by the light-absorbing material and ulti+ mately the screen is heated to the volatilization temperature of the film covering of the light-absorbing material.

BRIEF DESCRIPTION OF THE DRAWING The features of the present invention which are believed to be novel are set forth with particularity in the appended claims. The invention, together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawing, in the several figures of which like reference numerals identify like elements, and in which:

FIGS. 1 to 7 are similar fragmentary cross-sectional views of the faceplate of a color cathode-ray tube showing sequential process steps involved in screening a cathode-ray tube in accordance with one aspect of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT such material to its assigned portion on the screen for'the purpose of avoiding cross contamination.

The envelope of such a tube has a faceplate section and a complementary conically shaped section which are initially separated from one another as a convenience for screening. A fragmentary portion 10 of such a faceplate is represented in FIG. 1 and, after it has been made chemically clean, it is coated with a removable layer 11 of a photosensitive material having only volatilizable ingredients. It is convenient to form this layer of a clear PVA sensitized by ammonium dichromate which is normally soluble in water. After layer 11 of this material has been applied over faceplate 10, it is dried. The dried coating is then exposed to actinic radiation, such as ultraviolet light, to establish images of elemental areas of the screen that are separated from one another and are intended to receive deposits of phosphor materials. To that end, layer 11 is exposed through the shadow mask 12 of the tube in process in an exposure chamber or lighthouse of known construction including an energy source, such as a lamp and collimating system, which directs ultraviolet light to elemental areas of the screen through the transparent portions or apertures of the shadow mask. If the light source is positioned, for example, to simulate the electron gun of the tube in process which is to excite the green phosphor material, the ultraviolet light will expose only those portions 11g of layer 11 which overlie elemental areas of screen 10 assigned to receive deposits of green phosphor. After this exposure step, portions 11b which overlie elemental areas of screen 10 intended to receive deposits of blue phosphor are similarly exposed. This merely requires a change in the position of the light source so that it now simulates the electron gun of the tube that is to energize the blue phosphor dots. Finally, there is a third exposure with the light'source now simulating the red electron gun of the tube to expose portions lir which overlie elemental areas of the screen intended to receive red phosphor. By these multiple exposures, there are established in layer It images 113, 11b, llr of all those elemental areas of screen 10 that are to receive deposits of three phosphor materials. 7

It is to be noted that the exposed elemental areas 113, llb, llr of layer 11 are to be smaller in size than the apertures of the shadow mask as that mask is used in the completed tube. A convenient way of accomplishing this result is to coat the shadow mask with a removable material which closes down the size of the apertures of the mask to the desired dimensions. The mask with its apertures closed down in such fashion is used in the afore described exposure steps and in the screening of the various phosphor materials following which the coating of the mask is removed so that the apertures are restored to their original size and are larger than the phosphor dots formed on faceplate 10.

In the next processing step the images of the aforedescribed elemental areas 11g, llb, llr of screen 10 are developed, producing the screen condition of FIG. 2 in which the screen now bears only the portions of layer 11 that had been exposed. The development step is well known and, for the case under consideration in which layer 11 is a clear and sensitized PVA, is simply a matter of washing the screen with water to remove the unexposed portions of layer 11. In other words, the screen as shown in FIG. 2 has clear deposits of PVA separated from one another by screen areas which are bare and are to receive a material having light-absorbing capabilities. The processing is continued by depositing such material in the space between the elemental areas of the screen that are covered by dots of clear PVA. It is desirable, however, first to render the deposits of PVA opaque to actinic energy. This may be accomplished by the application of a dye, such as tartrazine yellow or congo red, or by baking the screen to a temperature of 175 C. or higher, but, of course, less than its decomposition temperature which is about 380 C. Baking at this temperature for approximately 30 minutes turns the PVA deposits brown so that they no longer are transmissive to ultraviolet light.

The screen is now prepared for the application of the lightabsorbing material in a photosensitive resist which preferably is also PVA sensitized with ammonium dichromate. The faceplate having received such a coating 13 is exposed by diffuse radiation through the surface opposite that which has been coated, as indicated by the arrows in FIG. 3. Since the dots 1 lg, 1 lb, 1 Ir of PVA have been rendered opaque, the exposure establishes in layer 13 an image of simply those surfaces which intervene the clear PVA dots and these images are developed by washing the screen with water.

Layer 13 may include ceramic black or other light absorbing pigment or, as described in U.S. Pat. No. 3,365,292 mentioned above, the layer may contain manganous carbonate which, at the time the screen is finally baked, is converted to manganese dioxide. While the carbonate is not as absorptive of light as desired, manganese dioxide is effective for use as black-surround material.

The screen following the last-mentioned developing step has the appearance represented in FIG. 4, featuring light-absorbing material 130 surrounding a multiplicity of dots 11g, 11b, llr of clear PVA which have protected the elemental areas of the screen intended to receive phosphors from the possibility of contamination that would ensue were the lightabsorbing material able to penetrate those areas during its application. It is highly desirable to preserve this state of affairs and protect the deposits 13a against erosion that may be experienced in the screening steps undertaken to deposit phosphor material over the dots of clear PVA. Should erosion occur, for example in response to the application of a phosphor slurry to faceplate 10, some particles of pigment will be released to migrate into areas 11g, 11b, llr and produce cross contamination. Accordingly, the invention contemplates forming over light-absorbing material 13a an overcoat of a nontacky and surface hardened film of volatilizable material that is insoluble in the slurry used in phosphor deposition. In selecting the material for overcoating the light-absorbing deposit 13a, regard must be given to its compatibility both from the standpoint of the remaining screening steps and the tube operation. Specifically, the overcoat must not adversely react with the materials employed in applying the various phosphors and in filming and aluminizing. Further, it must be volatilizable which is used here in the broad sense to encompass removal by conversion to the vapor state or by decomposition by heat treatment. The overcoat may be a film of acrylic or nitrocellulose lacquer or any organic resinous material which, when dried after being applied as an emulsion, becomes nontacky and exhibits a surface hardness. In particular, a thin film of sensitized PVA may be applied to the entire faceplate, dried, and then subjected to a flooding beam of ultraviolet light to be rendered insoluble in water and to acquire a greater degree of surface hardness. The overcoat affords a surface protection which prevents erosion and migration of particles of light-absorbing pigment during the application of the phosphor materials. If desired, where the overcoat is sensitized PVA it may be baked after its exposure to a temperature of about 275 C. for approximately l minutes for the purpose of further hardening its exposed surface.

The screen as shown in FIG. 5 with the volatilizable protective overcoat 14 is ready to receive deposits of red, blue, and green phosphor in accordance with any known coating process. For example, the phosphor may readily be applied by slurry screening with a coating composition of PVA, a sensitizer and phosphor particles of the appropriate color. The green phosphor is deposited exclusively on PVA dots 11g, while the blue phosphor is applied exclusively on PVA dots 11b, and the red phosphor is applied exclusively on the third set of dots llr. In FIG. 6 the green phosphor deposits are designated 11g, the blue phosphor deposits are designated 11b, and the red phosphor dots are llr. Each is shown deposited over film l4 and over one of the previously formed dots of clear PVA In the final baking process of the screen, subsequent to both the filming and aluminizing steps, all the volatilizable materials, including in particular PVA dots 113, Mb, llr and film 14, are removed, leaving the screen with dots 11 g', 11b, llr of phosphor material surrounded by deposits 13a of light-absorbing pigment as represented in FIG. 7.

By way of further particularization, an overcoat of PVA that has been employed successfully in practicing the invention was applied from a composition having the following ingredients:

200 grams of 10 percent PVA 200 cc. of methanol 200 cc. of water 40 cc. of ammonium dichromate. A slurry for applying an overcoat of nitrocellulose may be prepared of 2 percent solid nitrocellulose in amyl acetate with an equal volume of acetone.

Although there are advantages in applying the pattern of light-absorbing material first in processing a black-surround screen, if desired, the pattern of interleaved sets of phosphor deposits may be formed first. The general method is discussed in U.S. Pat. No. 3,146,368 referred to above. So far as forming the mosaic screen is concerned, it is simply a question of depositing phosphor dot triads over the screen area in any well-known manner, for example, by slurry screening. The only departure from conventional screening techniques is dimensioning the phosphor dots so that they are smaller in size than the apertures of the shadow mask and, as noted above, this may be readily accommodated by temporarily closing down the apertures of the shadow mask and using the mask with its reduced aperture size for screening. After the screening process has been completed, the material that has temporarily closed down the apertures of the mask is removed and the relative sizes of aperture mask to phosphor dots will have been established.

After the dot triads have been formed on the screen, the light-absorbing material may be applied to cover the elemental areas of the screen that surround the phosphor dots by a series of process steps very similar to those described in the discussion, supra, of FIGS. 3 and 4. In this case, however, the designations llg, 11b and llr will be understood to represent dots of phosphor material rather than deposits of clear PVA as in the first-described embodiment. The phosphor dots may be sufficiently opaque to ultraviolet light that exposure of layer 13 from the free or uncoated side of faceplate 10, as indicated by the arrows of FIG. 3, serves simply to establish images of the spaces that intervene the phosphor dots. The usual development step will then suffice to produce the screen condition of FIG. 4 which, for the case presently under consideration, represents phosphor dot triads surrounded by light-absorbing material.

It is beneficial, however, to form a film or overcoat of volatilizable material over the phosphor dots before applying layer 13 and, again, the material of this film is to be insoluble and nonmiscible with the composition employed in applying this layer or in developing the black-surround pattern. Such as overcoat should be nontacky and should exhibit a surface hardness so that the application of layer 13 does not introduce particles of pigment into the phosphor dots. This is a precaution against cross contamination and is effective whether the overcoat be a film of lacquer, such as nitrocellulose, or a thin layer of sensitized PVA which has been dried and exposed to actinic radiation to render it insoluble and to enhance its surface hardness.

An overcoat of this type is particularly desirable where the light-absorbing material is applied after the phosphor dots have been deposited as an ingredient of, or a subsequent coating upon, a photosensitive resist that is water soluble. It is, of course, convenient to apply the light-absorbing material by means of a similar vehicle which greatly facilitates the processing of the screen but does introduce the possibility of cross contamination since the water base of the carrier liquid for the light-absorbing material would tend to cause the exposed surfaces of the phosphor dots to become tacky. If this should happen, the possibility of cross contamination by particles of pigment being retained on the surface of the phosphor dots is evident. A previous approach to the use of an overcoat is described in U.S. Pat. No. 2,992,919, issued July 18, l96l to Beeler et al. but in that case the overcoat is a water soluble gelatin which, of course, is soluble in the slurry compositions. Preferred results in minimizing the cross contamination are obtained through the practice of the present invention which features an insoluble and volatilizable film, exhibiting surface hardness.

While particular embodiments of the invention have been shown and described, it will be obvious to those skilled in the art that changes and modifications may be made without departing from the invention in its broader aspects, and, therefore, the aim in the appended claims is to cover all such changes and modifications as fall within the true spirit and scope of the invention.

We claim:

1. The method of forming on the screen of a color image reproducing device a multiplicity of interleaved sets of deposits of different phosphor materials separated from one another by light-absorbing material, which method comprises the following steps:

coating said screen with a removable layer of a photosensitive material having only volatilizable ingredients and having a surface characteristic that may be altered by exposure to actinic energy;

exposing selected portions of said screen to actinic energy to establish in said layer latent images of elemental 'areas of said screen that are separated from one another and are to receive said phosphor materials;

developing said latent images of said elemental areas;

rendering said developed images opaque to actinic energy;

covering said screen and said developed images with a second layer of a photosensitive material which includes as ingredients a volatilizable photosensitive material and an inorganic pigment material having light absorbing capabilities and which has the property of becoming insoluble in a first solvent in response to exposure to actinic energy;

flooding said screen with actinic energy from the side thereof opposite that which is covered by said second layer;

treating said screen with said first solvent to remove all of said second layer that covers said developed images;

covering said screen with a film of volatilizable material that develops a surface hardness and becomes insoluble in a given solvent by exposure to actinic energy;

flooding said screen with actinic energy to surface harden said film;

coating said screen with another layer of a photosensitive material, including a volatilizable photosensitive material and a phosphor, and having a solubility in said given solvent which is a function of its exposure to actinic energy;

exposing said other layer with actinic energy to insolubilize the portions thereof which overlie said developed images;

treating said screen with said given solvent to remove unexposed portion of said other layer;

and baking said screen to remove the residual of said volatilizable materials from said screen.

2. The method in accordance with claim 1 in which said film is formed of nitrocellulose.

3. The method in accordance with claim 1 in which the solvents of said photosensitive layer and said film are the same. 

2. The method in accordance with claim 1 in which said film is formed of nitrocellulose.
 3. The method in accordance with claim 1 in which the solvents of said photosensitive layer and said film are the same. 